Trinucleotide repeat expansions have been shown to be the mutational mechanism responsible for a growing number of diseases, including Fragile X mental retardation, spinobulbar muscular atrophy, myotonic dystrophy (DM), Huntington disease (HD), spinocerebellar ataxia (SCA) types 1,2,3 and 6, dentatorubral pallidoluysian atrophy and Friedreich's ataxia. A hallmark for most of these diseases is the presence of anticipation, or a decrease in the age of onset and increase in disease severity in consecutive generations due to the tendency for the unstable trinucleotide repeat tract to lengthen when passed from one generation to the next (Warren, S. T. Science, 271, 1374–1375 (1996)).
In 1993, Schalling et al. (Nature Genetics, 4, 135–139 (1993)) developed the repeat expansion detection (RED) assay. RED is an elegant technique that detects potentially pathological trinucleotide repeat expansions without prior knowledge of chromosomal location or flanking DNA sequence. Human genomic DNA is used as a template for a two-step ligation cycling process that generates sequence specific [(CAG)n, (CGG)n, etc.] oligonucleotide multimers when expanded trinucleotide sequences are present in the genome. The assay was originally developed to detect very large trinucleotide repeat expansions present in genomic DNA from patients with Myotonic Dystrophy (DM) and Fragile X syndrome (up to 2,000 repeats). Since that time, Lindblad et al. have modified the procedure to detect smaller trinucleotide repeats in the size range (40–100 CAG repeats) pathologic for SCA1, SCA3, HD, and SBMA (Lindblad, K., et al., Nature Genetics 7, 124 (1994), Lindblad, K. et al., Genome Research, 6, 965–971 (1996)).
This modified assay has been used to establish correlations that suggest the involvement of CAG expansions in diseases such as SCA7 (Lindblad, K. et al., Genome Research, 6, 965–971 (1996)), bipolar affective disorder (Oruc, L. et al., Am J Hum Genet., 60, 732–735 (1997)) and schizophrenia (Maraganore, D. M., et al., Neurology, 47, (1996)).
The spinocerebellar ataxias (SCAs) are progressive degenerative neurological diseases of the nervous system characterized by a progressive degeneration of neurons of the cerebellar cortex. Degeneration is also seen in the deep cerebellar nuclei, brain stem, and spinal cord. Clinically, affected individuals suffer from severe ataxia and dysarthria, as well as from variable degrees of motor disturbance and neuropathy. The disease usually results in complete disability and eventually in death 10 to 30 years after onset of symptoms. The genes for SCA types 1, 2, 3 and 6 have been identified. All contain CAG DNA repeats that cause the disease when the repeat region is expanded. Little is known how CAG repeat expansion and elongation of polyglutamine tracts relate to neurodegeneration. The identification of the SCA7 gene would provide an opportunity to study this phenomenon in a new protein system.
The significance of identifying ataxia genes provides an improved method for diagnosis of individuals with the disease and increases the possibility of prenatal/presymptomatic diagnosis or better classification of ataxias. Most of the genes associated with repeat expansions in the coding region including the other SCA genes now identified, show no homology to known genes.